本文描述了HTTP协议使用Etag对内容进行缓存以便减少不必要的网络传输以提高运行效率的机制。后面用英文写，但我的英文不太好，如有错误之处，希望大家帮我纠正，我的联系方式在后面。
This article describes the HTTP procotol mechanism of using Etag to cache resources and resume file download in order to reduce unnecessory network transfer so the performance can increase.
73Please respect copyright.ＰＥＮＡＮＡAK7VfnJ6bQ
Section A. ETag
Etag is an identity of a resource, for example, we can use "10000_20241125150500" as the identity of a file where 10000 is the file size and 20241125150500 is the last modifed time of the file. We tell client browsers the etag of a file, then the browser cache the file and its etag, when next time the file is requested again, the browser can first just send the etag of the file to server, and if the server find the etag remain unchanged, then the browser can use the cache, no need to get the file from server once again.
The server send the etag of a resource by header like this:73Please respect copyright.ＰＥＮＡＮＡgworFRcyPX
---------------http response with etag----------------73Please respect copyright.ＰＥＮＡＮＡTSYrI897DJ
HTTP/1.1 200 OK73Please respect copyright.ＰＥＮＡＮＡI5yiY5smZX
Date: Mon, 25 Nov 2024 10:56:12 GMT73Please respect copyright.ＰＥＮＡＮＡxAMXuwwGct
accept-ranges: bytes73Please respect copyright.ＰＥＮＡＮＡCXRh3kFB4q
ETag: "534231200_1732277116"73Please respect copyright.ＰＥＮＡＮＡ1Ldr5KT5p3
Content-Length: 53423120073Please respect copyright.ＰＥＮＡＮＡwC14rHgzjq
Content-Type: application/octet-stream73Please respect copyright.ＰＥＮＡＮＡAFgybRcFo7
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)73Please respect copyright.ＰＥＮＡＮＡpRcN4C5B7q
-----------------------------------------------------
73Please respect copyright.ＰＥＮＡＮＡTG2TJVW3do
Section B. if-none-match
The browser send back the etag to ask server if it has been changed by header like this:73Please respect copyright.ＰＥＮＡＮＡYxWPWes2BK
-----http request for a cached resource------73Please respect copyright.ＰＥＮＡＮＡZ6VvxV0pYO
GET /zen.iso HTTP/1.173Please respect copyright.ＰＥＮＡＮＡaDRK7tyDyN
if-none-match: "534231200_1732277116"73Please respect copyright.ＰＥＮＡＮＡFcp9GAeMxz
---------------------------------------------
If the etag is unchanged, the server response like this:
1. for get and head requests:73Please respect copyright.ＰＥＮＡＮＡepUi3rTrjV
-----------http response for unchanged resource------73Please respect copyright.ＰＥＮＡＮＡwfz9jtxspX
HTTP/1.1 304 Not Modified
(http body canbe empty, the client will used its cache)73Please respect copyright.ＰＥＮＡＮＡgMJ5wLcr1Y
----------------------------------------------------
2. for other methods requests:73Please respect copyright.ＰＥＮＡＮＡKtjibOSIuo
-----------http response for unchanged resource------73Please respect copyright.ＰＥＮＡＮＡIH1fMoi31K
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will used its cache)73Please respect copyright.ＰＥＮＡＮＡkuXwSdfGnh
----------------------------------------------------
73Please respect copyright.ＰＥＮＡＮＡvHkM1gaiQ7
If the etag changes, the server response 200 OK with the new file content like this:73Please respect copyright.ＰＥＮＡＮＡjezI8FomFL
---------------http response with etag----------------73Please respect copyright.ＰＥＮＡＮＡ1fwOKYWCrH
HTTP/1.1 200 OK73Please respect copyright.ＰＥＮＡＮＡRetHbLo6tS
Date: Mon, 25 Nov 2024 10:56:12 GMT73Please respect copyright.ＰＥＮＡＮＡwwgVele5TO
accept-ranges: bytes73Please respect copyright.ＰＥＮＡＮＡ2qU8pUqTDm
ETag: "534231200_1732277116"73Please respect copyright.ＰＥＮＡＮＡsdEVch9dbg
Content-Length: 53423120073Please respect copyright.ＰＥＮＡＮＡfWiXhg3atA
Content-Type: application/octet-stream73Please respect copyright.ＰＥＮＡＮＡGLU2MQusGl
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)73Please respect copyright.ＰＥＮＡＮＡsdhwqwZArK
-----------------------------------------------------
73Please respect copyright.ＰＥＮＡＮＡKXi0IMvO0X
Section C. if-match
When resuming a download in the browser download manager, the browser sends a range request with the if-match header like this:73Please respect copyright.ＰＥＮＡＮＡ93A0faXEqD
------resume download request-------73Please respect copyright.ＰＥＮＡＮＡ3wtb9wjMpv
GET /zen.iso HTTP/1.173Please respect copyright.ＰＥＮＡＮＡQcdWawASaU
Range: bytes=287718144-73Please respect copyright.ＰＥＮＡＮＡsoK7m9hmb0
If-Match: "534231200_1732277116"73Please respect copyright.ＰＥＮＡＮＡSQRz8y3VbZ
-----------------------------------73Please respect copyright.ＰＥＮＡＮＡ7RNl7IZpkP
It tells the server to check the etag in If-Match, if unchanged, resume to get the remain content; otherwise, return 412 Precondition Failed.
The server response for unchanged etag is like this:73Please respect copyright.ＰＥＮＡＮＡpTg8bNjHy0
-------------range response-------------------73Please respect copyright.ＰＥＮＡＮＡknouf8mtt8
HTTP/1.1 206 Partial Content73Please respect copyright.ＰＥＮＡＮＡ4zoqkKjsIO
accept-ranges: bytes73Please respect copyright.ＰＥＮＡＮＡBscqBLS8lM
content-range: bytes 287718144-534231199/53423120073Please respect copyright.ＰＥＮＡＮＡeFXmUtcrCm
ETag: "534231200_1732277116"73Please respect copyright.ＰＥＮＡＮＡmk1ao8YaJ7
Content-Length: 24651305673Please respect copyright.ＰＥＮＡＮＡLYDusv6WR8
Content-Type: application/octet-stream73Please respect copyright.ＰＥＮＡＮＡqr9IjhvLvH
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)73Please respect copyright.ＰＥＮＡＮＡxQk4OU9ds6
-------------------------------------------
If the etag changes, response 412 like this:73Please respect copyright.ＰＥＮＡＮＡBcI0ep5z8l
-----------http response for changed resource------73Please respect copyright.ＰＥＮＡＮＡOBNjKfqW98
HTTP/1.1 412 Precondition Failed
(http body canbe empty, the client will stop downloading the resumed file)73Please respect copyright.ＰＥＮＡＮＡzqQAhJFa7z
---------------------------------------------------
73Please respect copyright.ＰＥＮＡＮＡqhNXCj9Qw7
Section D. if-range
If-Range is similar with If-Match, it's used by chrome when resuming a download in the download manager, whereas firefox use If-Match. They are the same except that when the etag changes, if-range response the resource content from beginning, so the resumed download will start downloading from beginning, while if-match just response the 412 error to stop the resumed download. I think chrome do a better job.
---resume download request by chrome---73Please respect copyright.ＰＥＮＡＮＡPCUK1VZL43
GET /zen.iso HTTP/1.173Please respect copyright.ＰＥＮＡＮＡUGCmXgwLGQ
Range: bytes=287718144-73Please respect copyright.ＰＥＮＡＮＡN0K5mRoSam
If-Range: "534231200_1732277116"73Please respect copyright.ＰＥＮＡＮＡ8tcMlOBdLL
------------------------------------73Please respect copyright.ＰＥＮＡＮＡngMIQ5Zq1G
It tells the server to check the etag in If-Range, if unchanged, resume to get the remain content; otherwise, return 200 OK and the file content from the beginning.
The server response for unchanged etag is like this:73Please respect copyright.ＰＥＮＡＮＡ1rFDht72AM
-------------range response-------------------73Please respect copyright.ＰＥＮＡＮＡ4lllY0uivA
HTTP/1.1 206 Partial Content73Please respect copyright.ＰＥＮＡＮＡFss72hgS13
accept-ranges: bytes73Please respect copyright.ＰＥＮＡＮＡ7KX9RNug68
content-range: bytes 287718144-534231199/53423120073Please respect copyright.ＰＥＮＡＮＡEKyyweQjiD
ETag: "534231200_1732277116"73Please respect copyright.ＰＥＮＡＮＡKIfqxm2vjg
Content-Length: 24651305673Please respect copyright.ＰＥＮＡＮＡblG0RSG3m4
Content-Type: application/octet-stream73Please respect copyright.ＰＥＮＡＮＡQwsN00ckhT
Content-Disposition: attachment; filename="zen.iso"
(http body is the remain file content)73Please respect copyright.ＰＥＮＡＮＡosS9GsCYeZ
-------------------------------------------
If the etag changes, response 200 from beginning like this:73Please respect copyright.ＰＥＮＡＮＡywMhkiD52Q
-----------http response from the beginning------73Please respect copyright.ＰＥＮＡＮＡ2qeJZ1cwTv
HTTP/1.1 200 OK73Please respect copyright.ＰＥＮＡＮＡL9p5A3Zgjo
Date: Mon, 25 Nov 2024 10:56:12 GMT73Please respect copyright.ＰＥＮＡＮＡS4epnD2htW
accept-ranges: bytes73Please respect copyright.ＰＥＮＡＮＡPQVzLlSwgR
ETag: "534231200_1732277116"73Please respect copyright.ＰＥＮＡＮＡz7sqmaPYgP
Content-Length: 53423120073Please respect copyright.ＰＥＮＡＮＡLBDM5xhDZO
Content-Type: application/octet-stream73Please respect copyright.ＰＥＮＡＮＡq1Il33b2s1
Content-Disposition: attachment; filename="zen.iso"
(http body for the resource)73Please respect copyright.ＰＥＮＡＮＡvC0HcweYjm
---------------------------------------------------
73Please respect copyright.ＰＥＮＡＮＡO794csCJeI
Section X. Thanks
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match73Please respect copyright.ＰＥＮＡＮＡnjOZ4KgYxW
https://blog.csdn.net/catoop/article/details/13417465373Please respect copyright.ＰＥＮＡＮＡvfCbotzQVc
https://blog.csdn.net/phker/article/details/50722619
Section Y. Contacts Me
If you found any errors or have any suggestions for this article, please let me know, my wechat: si_jinmin, my email: [email protected]73Please respect copyright.ＰＥＮＡＮＡscuE5aysHb
如果您发现本文有任何错误，或者对本文有好的建议，欢迎与我联系探讨，我的微信: si_jinmin, 我的email: [email protected]
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